SiPMs are semiconductor photon sensitive devices made up of an array of very small Geiger-mode avalanche photodiode (APD) cells on a substrate such as silicon. An example 10χ10 microcell array is shown in FIG. 1 of the accompanying drawings. Each cell is connected to one another to form one larger device with one signal output. The entire device size can be as small as 1χ1 mm or much larger. FIG. 2 of the accompanying drawings is a schematic diagram of an exemplary silicon photomultiplier.
APD cells vary in dimension from 10 to 100 microns depending on the mask used, and can have a density of up to 3000 microcells/sq. mm. Avalanche diodes can also be made from other semiconductors besides silicon, depending on the properties that are desirable. Silicon detects in the visible and near infrared range, with low multiplication noise (excess noise). Germanium (Ge) detects infrared to 1.7 μm wavelength, but has high multiplication noise. InGaAs (Indium Gallium Arsenide) detects to a maximum wavelength of 1.6 μm, and has less multiplication noise than Ge. InGaAs is generally used for the multiplication region of a heterostructure diode, is compatible with high speed telecommunications using optical fibres, and can reach speeds of greater than Gbit/s. Gallium nitride operates with UV light. HgCdTe (Mercury Cadmium Telluride) operates in the infrared, to a maximum wavelength of about 14 requires cooling to reduce dark currents, and can achieve a very low level of excess noise.
Silicon avalanche diodes can function with breakdown voltages of 20 to 500V, typically. APDs exhibit internal current gain effect of about 100-1000 due to impact ionization, or avalanche effect, when a high reverse bias voltage is applied (approximately 20-200 V in silicon, depending on the doping profile in the junction). Silicon Photomultipliers or SiPMs can achieve a gain of 105 to 106 by using Geiger mode APDs which operate with a reverse voltage that is greater than the breakdown voltage, and by maintaining the dark count event rate at a sufficiently low level. The current generated by an avalanche event must be quenched by an appropriate current limited scheme so that the device can recover and reset after an avalanche event.
In existing avalanche diodes light interacts with a silicon lattice to generate electron hole pairs which cause breakdown at a junction where there is a peak electric field. In the Geiger mode, there is a high applied bias that accelerates the electrons and holes causing impact ionisation and avalanche breakdown. It is desired that this effect only occur by electrons and holes initiated by photons. However, minority carrier current exists which cause spurious counts, known as the dark count, in the absence of light. This is undesireable. There is therefore a need for an avalanche photodiode and a process of manufacturing an avalanche photodiode which addresses at least some of the drawbacks of the prior art.